The annular ring (Ar) is the 2D ring that’s surrounding a plated through hole. It extends from the edge of the hole to the edge of the pad.

This is important to know because this ring has a minimum. You will be asked to change the design if the minimum is not met. MIL-STD-275E and IPC class 3 calls for a minimum annular ring of 0.05mm (0.002").

Some PCB manufacturers require a minimum of 6mil between the edge of the hole to the edge of the pad to allow for drilling intolerances. Laser drilling applications require 4mil annular rings.